1. Field of the Invention
The present invention relates to an image forming apparatus, such as a printer and a copying machine.
2. Description of the Related Art
There have been known the following techniques for calibrating an output image to be output from an image forming apparatus, such as a copying machine or a printer.
A specific test pattern is formed on a recording sheet by the image forming apparatus, and then image information, such as density or chromaticity, of the test pattern formed on the recording sheet is read by an image reading means. Thereafter, density correction, gradation correction, and the like correction are performed based on the image information, whereby image quality is adjusted to desired characteristics, and its stability is enhanced (see e.g. Japanese Patent Laid-Open Publication No. S62-296669, and Japanese Patent Laid-Open Publication No. S63-185279). This method is called calibration.
In the case of an electrophotographic image forming apparatus, for example, an image calibration method is known in which a charge bias voltage and a developing bias voltage are controlled to thereby change a latent image contrast potential and a developing contrast potential so as to perform correction such that image density is maximized (image density-maximizing correction). Further, an image calibration method is also known in which a gradation correction table is changed so as to correct gradation characteristics.
However, in the conventional image density-maximizing correction, the developing contrast potential is controlled so as to obtain the maximum density, but it often occurs that the resulting developing contrast potential does not match with a developing contrast potential for achieving high image quality.
In the electrophotographic image forming apparatus, through the use of high contrast, it is possible to stabilize an electrostatic process and enhance image reproducibility, and hence it is sometimes desired to determine a contrast potential irrespective of the maximum density. In this case, adjustment of the maximum density is performed using a γ correction table (γ correction circuit), which can cause reduction of the number of gradations depending on the amount of adjustment.